1. Field of the Invention
The present invention relates to a process for making high purity, particulate silica gel from a silicon alkoxide starting material.
2. Description of the Prior Art
In general, there are two ways in which silica gel can be manufactured.
The first starts with a naturally occurring inorganic silica source, such as sodium silicate, which is treated with acid in water to precipitate silica as well as certain impurities, e.g., sodium cation, and the anion of the acid. In general, a washing procedure would be used in order to remove such contaminants from the silica gel product. In a recent reference (Chem. Abstr. 108:58828) another procedure was described in which a silica source was dissolved in a mixture of hydrofluoric acid and sulfuric acid, distilled, and the distillate, comprising water and silicon fluoride, was mixed with aqueous ammonia to form silica gel which was filtered, washed with water, dried and then fired to give high purity silica.
An alternative way of forming silica gel relies upon the use of a silicon alkoxide (or organic-containing) starting material. Such a material, if acidified with aqueous acid, forms a gel upon ageing. Treatment of the acidified solution with a base, such as ammonium hydroxide, accelerates formation of the gel, a product which theoretically can have a much higher purity level than silica gel formed from the naturally occurring inorganic sodium silicates, for example. A general article describing such an approach, which does not specifically address how a high purity, particulate product might be obtained, is "Silicon Alkoxides in Glass Technology" by L. C. Klein et al., ACS Symp. Series 194, Chapter 18, pp. 293-304 (1982). In Japanese Patent Publication No. 60/239,329 a sol-gel process for forming high purity silica monoliths (rather than powders) is described in which powdered silica is added with the alkoxide during the hydrolysis step, which is followed by a centrifuging step to remove large silica particles. The slurry is then gelled, and is washed and dried. It appears that the procedure is a bench scale experiment that involves transfer of product between various reaction vessels. In general, persons of ordinary skill in the art who use silicon alkoxides in a sol-gel synthesis route deem that certain precautions are essential if a high purity silica gel is to be formed. The synthesis is, for example, carried out under clean room conditions using either quartz glass or plastic apparatus with highly pure reagents. The use of special quartz glass or plastic in the reactor vessels and related equipment is deemed essential in precluding the presence of trace levels of contaminants such as alkaline metal cations, metallic cations, and the like. For example, R. G. Gossink et al. in Mat. Res. Bull., Vol. 10, pp 35-40 (1975) indicate use of quartz glass and TEFLON fluoropolymer in the apparatus used for hydrolysis of ethyl silicate in a laminar floW box under clean room conditions. Japanese patent publication No. 62/91,428 also mentions use of clean room conditions in conjunction With the sol-gel method to form glass which is apparently in the form of a monolith rather than being in particulate form. Unfortunately, the use of such clean room conditions and specialized equipment can render the synthesis procedure prohibitively expensive. Although it may be possible to use such conditions and specialized equipment for lab scale preparations, the economic cost for using such conditions and equipment largely precludes the scale-up of the procedure to either pilot plant or plant scale.
Japanese Patent Publication No. 62/59,515 indicates hydrolysis of tetramethoxysilane in the presence of membrane filter-purified carbon dioxide in either a stainless steel or glass reactor which does not contaminate the product with any impurities from the reactor.
German Patent Publication No. 3,500,080 avoids hydrolysis of tetraethoxysilane in the production of high purity silica, choosing instead its oxidation in an inert gas stream (e.g., N.sub.2) in a cold plasma which contains oxygen.
Su et al., in Mat. Res. Soc. Symp. Proc., Vol. 73, pp. 237-244 (1986) and in U.S. Pat. No. 4,680,049 describes the synthesis of high purity silica glass (in monolithic form, rather than particulate form) from the metal alkoxide and utilize supercritical drying of the gel produced in their process.